Using the plural pronoun to refer to a singular antecedent is not really a modern novelty. It has been acceptable in English for centuries:
"And whoso fyndeth hym out of swich blame, They wol come up . . ." —Chaucer, The Pardoner's Prologue (c. 1395)
" 'Tis meet that some more audience than a mother, since nature makes them partial, should o'er hear the speech." — Shakespeare, Hamlet (1599)
"If a person is born of a . . . gloomy temper . . . they cannot help it." — Chesterfield, Letter to his son (1759)
The "rule" that the use of "he" is to be used as the only acceptable pronoun for a singular antecedent began to be established in the 19th Century in much the same way as the "rule" that one cannot use a preposition to end a sentence with [sic]. English speakers are merely reverting, unknowingly, to the way the English language originally developed.
The "he/she" construction, however, is crap established by the politically correct crowd.